How To Rotate some Vertices around a changed Pivot Point?
If you don't want the shoulder to be part of the selection, you should use the 3D cursor. To place the 3D cursor at the origin of the object you want to orient about, click the object (or vertex, it works in edit mode too) and Shift+S to open the cursor menu.



Then you can select Cursor to Active, this will place the 3D cursor at the origin of the selected object. Then change the Pivot Point to 3D Cursor, and whatever you move/rotate will orient around the 3D Cursor.